Effect of Protonation and Zn(II) Coordination on the Fluorescence Emission of a Phenanthroline-Containing Macrocycle. An Unusual Case of “Nonemissive” Zn(II) Complex

Abstract

Ligand 2,5,8-triaza[9]-10,23-phenanthrolinophane (L) contains a triamine chain connecting the 2,9 positions of a phenanthroline unit. Protonation of L has been studied by means of potentiometric and 1H and 13C NMR techniques, allowing the determination of the basicity constants and of the stepwise protonation sites. Protonation strongly affects the fluorescence emission properties of the chemosensor L. The two benzylic amine groups, namely, the two aliphatic amine groups adjacent to phenanthroline, are the most efficient nitrogens in fluorescence emission quenching. In the diprotonated receptor [H2L]2+ both of these nitrogens are protonated, and therefore this species is the most emissive. In the [H3L]3+ species the three acidic protons are located on the amine groups of the polyamine chain. This species is still emissive, but less so than [H2L]2+, due to formation of a hydrogen bond network involving the phenanthroline nitrogens, as shown by the crystal structure of the [H3L]Br3·H2O salt. A potentiometri...